Recovery of pure hydrocarbons



Nov. 2, 1954 s. H. HASTINGS El AL 216931495 RECOVERY OF PURE HYDROCARBONS Filed Feb. 15, 1952 2 BENZENE FEED 1 1 1 A '7 XYLENES FEED 9 L GEL LIGHT I 4 SATURA TES 1 asuznvs $LOP our FIG. 2. 1

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United States Patent Ofiice 2,693,495 Patented Nov. 2, 1954 RECOVERY OF PURE HYDROCARBONS Sam H. Hastings, James A. Anderson, Jr., and Albert T.

Watson, Baytown, Tex., assignors, by mesne assignments, to Standard Oil Development Company, Elizabeth, N. J., a corporation of Delaware Application February 15, 1952, Serial No. 271,792

1 Claim. (Cl. 260-674) The present invention is directed to a method for recovering pure hydrocarbons. More particularly, the invention is directed to a method for separating one type of hydrocarbon in admixture with another type of hydrocarbon. In its more specific aspects the invention is directed to a cyclic adsorption process in which a hydrocarbon in admixture with a hydrocarbon of a ditferent type is recovered substantially completely in purified form from the admixture.

The invention may be briefly described as involving a cyclic adsorption process for separating hydrocarbons of difierent types which comprises contacting a first feed stream containing at least a first desired hydrocarbon and an undesired hydrocarbon of similar boiling points and difierent types with a bed of a porous adsorbent under conditions to adsorb selectively the first desired hydrocarbon. The first feed stream is then replaced with a second feed stream containing at least a second desired hydrocarbon and an undesired hydrocarbon of the same types of the hydrocarbons in the first feed stream but having boiling points different from the hydrocarbons in the first feed stream under such conditions to desorb the first desired hydrocarbon and to adsorb the second desired hydrocarbon. The volume ratio of the first feed stream to the porous adsorbent is maintained at a value no greater than 2 such that there is obtained from the bed at least a product stream consisting substantially of the desired hydrocarbon from the first stream and hydrocarbons from the second stream. Thereafter the steps of the cycle are repeated.

The present invention is based on a discovery that by employing the first feed stream in the above described process in a critical amount, it is possible to recover substantially all of the desired hydrocarbons in substantially 100% purity. Of course, it is appreciated that the volume of the porous adsorbent and the concentration of the desired hydrocarbon in the feed are factors to be considered and by judicious choice of the volume ratio of the first feed stream to the porous adsorbent for a given concentration of the desired hydrocarbon in the first feed stream, it is possible to recover completely the desired hydrocarbon in 100% purity.

The invention is broadly directed to separating hydrocarbons of difierent types from each other. For example, the invention may be employed to separate and recover an aromatic hydrocarbon from a non-aromatic hydrocarbon. The invention is also applicable to separating a naphthenic hydrocarbon from a non-naphthenic hydrocarbon.

The feed stocks of the present invention may broadly include aromatic hydrocarbons ranging from those in the gasoline boiling range up to and including those in the lubricating oil boiling range. Suitable aromatic hydrocarbons which may be recovered in purified condition in accordance with the present invention include benzene, toluene, the xylenes, ethylbenzene, propylbenzene and the higher members of the homologous series including durene and the like as well as naphthalene and other condensed ring aromatics. 1

The aromatic hydrocarbons usually occur in admixture with paraflins, naphthenes, and/ or olefins. The nonaromatic hydrocarbons even in narrow cut fractions tend to form azeotropes with aromatic hydrocarbons which makes a separation by distillation difiicult except as provided in the present invention. While the aromatic hydrocarbons such as those illustrated may be recovered from a mixture of olefins it is preferred to recover the aromatic hydrocarbons from admixture with the paraffins and/ or naphthenes. Thus the feed stock may consist of or comprise essentially aromatics, paraflins and naphthenes. It must be understood, however, that we do not wish to limit ourselves to such a feed mixture since the invention is operable with mixtures of aromatics and naphthenes, aromatics and paraflins, naphthenes and olefins and the like. A preferred feed stock may be a mixture of benzene with paraflins and naphthenes having 6 and 7 carbon atoms in the molecule and xylenes with saturated hydrocarbons having 9 carbon atoms in the molecule.

The second aromatic hydrocarbon feed employed in the present invention will be an aromatic hydrocarbon having a boiling point diiferentfrom the first aromatic hydrocarbon feed. For example, when benzene is to be recovered from an admixture the second aromatic hydrocarbon feed may include toluene, the xylenes or the higher members of the same homologous series. If toluene or xylenes are to be recovered in substantially purified form, the second feed mixture may contain an aromatic hydrocarbon, such as benzene, or, in the case of toluene, xylenes. Similarly, the substituted benzenes having a boiling point diiferent from the aromatic hydrocarbons in the first feed stream may be employed as the second feed stream or as the desorbing agent as it may sometimes be termed.

The porous adsorbent employed in the practice of the present invention is preferably silica gel of 28 to about 250 mesh although silica gel passing mesh sizes up to as high as 350 may suitably be used. Silica gel is a well-known article of commerce and further description thereof is not deemed necessary. The invention is also operable using activated carbon or activated alumina, or other adsorbents having adsorptive power for hydrocarbons. The activated chars obtained from coconut and various other starting materials used for making activated carbon will give satisfactory results in the practice of our invention. Activated carbon may be made in a large number of different ways well known to the art and it is contemplated that the activated carbon of commerce may be employed. Of course, it is understood that the type of adsorbent will be selected for the type of hydrocarbon which is to be adsorbed preferentially. Silica gell may be preferred when aromatic hydrocarbons are to be preferentially adsorbed.

On the other hand, when naphthenic hydrocarbons are to be recovered in substantially purified form, it may be desirable to employ another porous adsorbent, such as activated carbon. The activated carbon of commerce may be used and in such cases the non-naphthenic hydrocarbons, such as paraflins, will be adsorbed selectively by the activated carbon and the naphthenes will pass through the bed substantially unadsorbed. The activated carbon may be an activated carbon or charcoal made from a large number of well known substances, such as bagasse, coconut shells, walnut pits, cereals, animal blood, bones, kelp, seaweed, petroleum sludges, acid petroleum sludges and many other materials too numerous to mention here. Suffice to say that the activated carbon should be in a condition to adsorb selectively the non-naphthenic hydrocarbons.

The activated carbon may suitably have a mesh size ranging from about 8 up to about 200 mesh.

The porous adsorbent may also be an activated alumina such as is well known on the market. The activated alumina should be selected to have a mesh size in the range from about 28 to about 250 mesh. The activated alumina is in condition such that it will adsorb selectively hydrocarbons by type.

The contact time at which the hydrocarbon feed mixture will be in contact with the porous adsorbent ordinarily will be at least about 15 minutes and may be as high as 2 hours or more. The time for flowing the several feed mixtures through the bed of porous adsorbent will depend, of course, on the volume of the feed mixture employed. For example, by sizing the amount of the first feed mixture to obtain substantially complete recovery and purity of the aromatic hydrocarbon in the feed mixture, it is possible to provide a contact time of lesser duration than that for the second feed mixture when the second feed mixture is employed in an amount greater than the first feed mixture. In any event, about 15 minutes to about 2 hours will suffice.

Temperatures to be employed may sultably range from about 32 F. to about 120 'F. with a temperature below 100 F. preferred. It may be desirable to employ cooling means within or extraneous to the bed to compensate for the heat liberated by adsorption of the aromatic hydrocarbons.

It is desirable that the bed be an elongated bed having a ratio of length to diameter in the range from about 2:1 to about 200:1. Actually, satisfactory results may be obtained with beds having length to diameter ratios of about 1:1, provided the present invention is practiced in which a volume ratio of the first stream to the porous adsorbent is no greater than 2 and is in the range from 0.1 to 2. The volume ratio of the second feed mixture to the porous adsorbent may range from about 3:1 to about 0.10:1. However, the volume ratio of the second feed mixture to the porous adsorbent will depend to a large extent on the volume ratio of the first feed stream to the porous adsorbent and will be selected to desorb completely the adsorbed hydrocarbons from the first feed stream. Stating this otherwise, the amount of the second feed stream should be sufiicient to allow substantially complete recovery of the adsorbed hydrocarbon from the first feed in substantially complete purity.

The concentration of the hydrocarbon to be recovered in the first feed stream, for example, aromatic hydrocarbons may range from 5% to as high as 65% by volume. Ordinarily it will be more desirable to recover aromatic hydrocarbons in accordance with our invention from low purity feed stocks than from high purity feed stocks since conventional techniques, such as solvent extraction, may be employed satisfactorily with the latter. However, it is to be understood that the process may be employed satisfactorily with high concentration aromatic feeds but economic consideration may place a ceiling on the upper limits.

The invention will be further illustrated by reference to the drawing in which Fig. l is a plot showing the relationship between the volume ratio of feed to silica gel and the percentage of benzene in the feed; and

Fig. 2 is a flow diagram illustrating a preferred mode of the invention.

The data from which Fig. 1 is constructed show the volume ratio of feed to silica gel plotted against the percentage of benzene in an aromatic containing first feed. The data from which Fig. 1 is constructed are presented in the table.

Table Volume Ratio of Feed to Silica Gel Percent Aromatics The data presented in the table illustrate the volume ratios required to obtain 100% recovery of 100% purity for a given percent of benzene in the first feed stream. Purified benzene may be obtained at ratios lower than those shown for a given purity feed but the throughput through the bed of porous adsorbent is sacrificed. Stating this otherwise, the bed of porous adsorbent, such as silica gel, is not utilized eificiently or completely. Operating above the ratios presented in the table required for a given purity of feed results in impurity or loss of the product; thus the benzene is not completely recovered from the adsorbent or the recovered benzene is not as pure as could be obtained at the critical ratio.

Referring now to Fig. 2, numerals 11 and 12 designate, respectively, a storage tank containing a benzene feed containing a non-aromatic hydrocarbon and a xylenes feed which likewise contains non-aromatic hydrocarbons. The xylenes feed in tank 12 is withdrawn therefrom by line 13 containing pump 14 and valve 15 into manifold 16 from whence it is routed by connecting line 17 into an adsorption zone 18 which contains a bed 19 of a porous adsorbent such as silica gel. The xylenes feed percolates downwardly in the bed 19 under conditions such that the xylenes are adsorbed selectively or preferentially and the non-aromatics in the xylenes feed tend to accumulate in the lower portion of the bed 19. When the bed 19 has become substantially completely filled with adsorbed xylenes and other hydrocarbons from tank 12 flow from tank 12 is interrupted by closing off valve 15 and stopping pump 14. Thereafter fiow from tank 11, which includes the benzene feed, is begun through line 20 containing pump 21 and valve 22. Line 20 likewise connects into manifold 16 and into line 17 The benzene feed is then routed through line 20, manifold 16 and line 17 into adsorption zone 18 such that the benzene is adsorbed selectively by the silica gel and the non-aromatic, which in this case is hexane, rlows downwardly in the bed. As a result of the flow of the benzene through the bed there is discharged from bed 19 heavy saturates which are withdrawn by line 23 and are routed by line 24 controlled by valve 25 into tank 26 where the heavy saturated material originally in the feed in tank 12 is accurulated. Thereafter the flow from tank 11 is discontinued by closing oif valve 22 and shutting down pump 21 and flow from tank 12 is resumed by opening up valve 15 and starting up pump 14 allowing the flow to proceed through line 13, manifold 16 and line 17. As the xylenes feed containing xylenes and heavy saturates flow downwardly through the bed 19 the benzene adsorbed on the silica gel is desorbed by the xylenes feed resulting in the benzene being pushed ahead through the column and, in turn, desorbing previously adsorbed xylenes and pushing the hexane originally in admixture with the benzene and desorbed xylenes outwardly from adsorp-. tion zone 18 by line 23. The light saturates are routed by line 27 controlled by valve 23 into tank 29 where the light saturates, including some xylenes, are accumulated. The benzene follows the light saturates and xylenes fraction and is routed by line 23 through line 2 controlled by valve 25 into tank 26, valve 28 in line 27 being closed. Thereafter valve 15 in line 13 is closed and pump 14 is stopped and pump 21 is started. Valve 22 in line 20 is opened allowing the benzene feed to flow through line 20, manifold 16 and line 17 into adsorption zone 18, resulting in the benzene being adsorbed selectively on the silica gel and the xylenes previously held therein to be desorbed. As a result there issues from adsorption zone 18 by line 23 a slop cut containing xylenes, some heavy saturates and light saturates which are routed through line 27 controlled by valve 28 into line 30 controlled by valve 31 to be discarded from the system or to be used as may be desired. During this latter operation, valve 32 in line 27 is in the closed position. After the slop cut is discarded a fraction is withdrawn from line 23 into line 27 and is discharged into tank 29, valves 28 and 32 being opened and valve 31 being closed. This fraction contains light saturates originally present in the benzene feed in tank 11 and xylenes from the xylenes feed in tank 12. Thereafter the cycle of operations as described is repeated with the benzene feed flowing alternately to adsorption zone 18 with the xylenes feed resulting in the obtaining in tank 26 of an admixture of benzene, xylenes and heavy saturates and of a slop cut of xylenes, heavy saturates and light saturates and another fraction of light saturates and xylenes allowing the recovery of benzene and xylenes.

It is to be noted, however, that we have eliminated a slop cut in which benzene would be discarded in accordance with the prior art techniques by virtue of our inventron which is maintaining a ratio of the first feed stream to the porous adsorbent no greater than 2 and in the range from 0.1 to 2.0.

Once the streams have been accumulated in tanks 26 and 29 in the fashion indicated, it is possible to recover substantially pure benzene substantially completely and xylenes in substantial purity. T o accomplish the recovery of benzene, the material accumulated in tank 26 is withdrawn by line 33 containing pump 34 and discharged into a distillation tower 35 which is provided with a heating means, such as coil 36, to allow separation between the benzene and the xylenes and heavy saturates. It is understood, of course, that distillation tower 35 is provided with all auxiliary equipment required for precise distillation including internal vapor-liquid contacting means, such as bell cap trays and the like, means for inducing reflux, and condensing means, to allow recovery as an overhead fraction by line 37 of substantially all of the benzene originally contained in the feed in tank 11 in purified form. The xylenes and heavy saturates are withdrawn from distillation tower 35 by line 38. Similarly, the product in tank 2') which contains light saturates and a substantial amount of xylenes but not all of them, since some were discarded by line 30 and some appear in line 38, is withdrawn by line 39 containing pump 40 and discharged into distillation tower 41 which is similar to distillation tower 35. Distillation tower 41 is provided with heating means illustrated by coil 42 which allows recovery of light saturates by line 43 and recovery of xylenes by line 44.

From the foregoing brief description taken with the drawing it will be clear that we have described a cyclic adsorption process which allows the obtaining of a desired hydrocarbon contained in a first of two or more feed streams in substantially complete purity and substantially complete yields without loss of the desired product in a slop cut as has been required heretofore in the prior art process. In other words, by suitably sizing the amount of the first feed stream in relationship to the porous adsorbent we are able to eliminate at least a slop cut and recover a fraction from which a desired hydrocarbon may be recovered completely and in substantially pure condition.

Referring now again to Fig. 1, it will be clear from an examination of this figure that it is possible to provide an operating technique whereby the operator having a given concentration of benzene, for example, in a particular feed which he wishes to recover substantially completely and in purified form, he need only consult the figure to decide what ratio of feed to silical gel he must employ. For example, if he has a feed containing benzene in admixture with hexane he will employ a ratio of feed to silica gel of 1. If he has a feed containing 30% benzene, for example, he would employ a ratio of feed to silica gel of 0.5. Likewise, if the feed contains only 5% benzene in admixture with hexane he would employ a ratio of feed to silica gel of 1.4. By operating at a critical ratio in accordance with the relationship of Fig. 1,

it is possible to recover the hydrocarbon desired in purified form in substantially complete yields.

The nature and objects of the present invention having been completely described and illustrated, what we wish to claim as new and useful and to secure by Letters Patent is:

In a cyclic method for separating an aromatic hydrocarbon from a non-aromatic hydrocarbon in which a first feed containing an absorbable amount of benzene in the range between 5% and by volume and the remainder non-aromatic hydrocarbons having six carbon atoms in the molecule is contacted with a bed of silica gel to adsorb selectively said benzene in the first feed and in which a second feed containing Xylenes and non-aromatic hydrocarbons having nine carbon atoms in the molecule and having boiling points difierent from that of the hydrocarbons in the first feed replaces said first feed in contacting said bed to adsorb selectively the aromatic hydrocarbon in the second feed and to desorb the adsorbed benzene from the first feed, the step of maintaining a critical volume ratio of the first feed to the silica gel for a given concentration of benzene in the first feed as read from Fig. 1 which is a plot of data showing the relationship of the ratio of the volume of first feed to porous adsorbent to the concentration of benzene in the first feed, whereby benzene is recovered substantially in purified form from the first feed.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,441,572 Hirschler et al May 18, 1948 2,449,402 Lipkin et a1 Sept. 14, 1948 2,564,717 Olsen Aug. 21, 1951 2,576,525 Lipkin Nov. 27, 1951 OTHER REFERENCES Eagle et al., Journal Ind. and Eng. Chemistry, vol. 42, July 1950, pages 1287 to 1293. 

